The number of frequency bands utilized in a mobile communication system has been increased. Recently, in the mobile communication system, a service is provided in multiple bands (multiband). More specifically, the service is provided in a 700 MHz band, an 800 MHz band, a 1.5 GHz band, a 1.7 GHz band, a 2.1 GHz band, and a 2.5 GHz band in the mobile communication system.
An amplifier used for a base station in the mobile communication system is requested to have a high efficiency performance. To satisfy the request of the high efficiency performance, the Doherty amplifier is adopted in many cases. The Doherty amplifier includes a carrier-amplifier and a peak-amplifier arranged in parallel. The carrier-amplifier regularly operates, and the peak-amplifier operates only at the time of a high output.
In the base station, an amplifier is prepared for each frequency band. However, the preparation of the amplifier for each frequency band is not preferable from viewpoints of a design, a cost, and an amount of resources. Therefore, a Doherty amplifier that can cope with the multiband by a single amplifier is desired.
A technique for achieving the high efficiency performance with respect to the multiband by switching, using a switch, an electrical length of an output power combining circuit of the Doherty amplifier in accordance with the frequency band is proposed (for example, see Japanese Laid-open Patent Publication No. 2006-345341).